Packet scheduling

ABSTRACT

A method of scheduling packets in a mobile communication system includes, in each time period, assigning to each bearer of a terminal, a credit. In each time period for which resources are available, accepting data packets from a bearer in accordance with predefined constraints of priority, buffer content and credit. An accepted data packet size may exceed the available credit by an amount in a predefined range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of International ApplicationNo. PCT/GB2007/050608, filed Oct. 3, 2007 and claims the benefitthereof. The International Application claims the benefits of UnitedKingdom Application No. 0619519.2 filed on Oct. 4, 2006 and UnitedKingdom Application No. 0621243.5 filed Oct. 26, 2006. All threeapplications are incorporated by reference herein in their entirety.

BACKGROUND

Described below is a method of scheduling of packets in a mobilecommunication system.

In mobile radio systems, where dynamic assignment of radio capacityamongst user equipments (UEs) operates in the uplink, for exampleuniversal mobile telecommunication system (UMTS) enhanced uplinkdedicated channel (E-DCH) transmission, for efficiency reasons, radioresources that are to be used within a time period can be assigned on aUE basis, rather than on the basis of the individual bearers that a UEis using to transmit data. This then requires that the UE apply a ruleregarding how much capacity is assigned to each bearer within the timeperiod, for example, between bearers assigned to signalling, speech andbest effort data services.

For UMTS E-DCH the rule that has been used is the following. For eachtransmission opportunity, i.e. transmission time interval (TTI), thescheduler allocates capacity to the highest priority bearer that has anon-empty transmission buffer. The principle is highest priority first,but unfortunately, at times of high cell load, when the UE may not beassigned all of the capacity that the data buffered for all bearersrequires, this can lead to starvation of the lower priority bearers.

For UMTS long term evolution (LTE) uplink transmission there is a desirethat starvation be avoided and greater flexibility in resourceassignment between bearers be possible, whilst retaining the per UE,rather than per UE bearer, resource allocation.

One proposal for assigning capacity has been to assign resources to eachbearer, in priority order, until each has received an allocationequivalent to the minimum data rate for that bearer, after which anyadditional capacity is assigned to bearers in, for example, priorityorder.

SUMMARY

In accordance with a first aspect, a method of scheduling packets in amobile communication system includes, in each time period, assigning toeach bearer of a terminal, a credit; in each time period for whichresources are available; and accepting data packets from a bearer inaccordance with predefined constraints of priority, buffer content andcredit; wherein an accepted data packet size may exceed the availablecredit by an amount in a predefined range.

In the method, a credit is held allowing maximization of size of apacket that is sent to reduce segmentation, so whole service data unitsare transmitted, thereby avoiding unnecessary segmentation. The use of acredit allows transmission of a whole packet, where normally there wouldonly be permission for transmitting a part of the packet.

The constraints of priority, buffer content and credit may be for thehighest priority, non-empty buffer and positive credit.

If the bearer satisfies the constraints, the content of the buffer maybe added to a data block for transmission to the limit of the credit, orof the next service data unit boundary.

The predefined range may be set to accommodate un-segmented packets.

Any amount by which the credit is exceeded may be carried forward to thenext time interval as a negative credit.

Any amount by which the credit is unused may be carried forward to thenext time interval as a positive credit.

An upper limit of carried forward positive or negative credit may beset.

The method may be subject to the proviso that the credit is not exceededby more than a set margin, and also that the whole SDU fits within thetransport block that is transmitted and equal in size to the grant.

The upper limit may be an integer multiple of the amount of creditallocated to a bearer in one time interval.

In any time interval, for which resources are available, each bearer maybe served at its minimum data rate in preference to bearers usingcarried forward credit.

The mobile communication system may be a universal mobiletelecommunication system.

In accordance with a second aspect, a method of controlling allocationof resources in a terminal of a communication system includes signallingfrom a network control entity to a mobile device, a time period; andusing the time period in a method according to the first aspect.

A timer is transferred to the mobile device, typically a UE, for use inregulating the achievement of meeting minimum bit rates, although if theUE sets its own internal timer, then credits can be used withoutadditional signalling.

The timer value may be transferred to the UE across the radio interfacein radio resource control signalling sent by the network, or the timeris set internally in the UE to a specified value.

For example, the timer may be sent by the base station (eNB in UMTSLTE), or the timer may be set to a value specified in a relatedstandardization specification.

The timer may be used within the UE to ensure that it meets minimum bitrate and/or maximum bit rate criteria set for the bearer.

The timer may be used in a UE test regime as a measure of the intervalin which the UE must meet the minimum and/or maximum bit rates set forthe bearer.

The mobile device may be a terminal of the communication system, or atesting entity used to verify the operation of the terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages will become more apparent andmore readily appreciated from the following description of the exemplaryembodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a flowchart of how a timer T can be used to increment creditsthat are subsequently used in the scheduler;

FIG. 2 is a block diagram of a time value T being delivered from thenetwork to the UE; and,

FIG. 3 is a flowchart of a scheduler using credits to decide how much ofan allocated grant is assigned to a bearer in a scheduling period.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments,examples of which are illustrated in the accompanying drawings, whereinlike reference numerals refer to like elements throughout.

The method enables flexible and minimum bit-rate criteria can beimplemented as follows. In each time period, T, which may be larger thanor equal to a TTI, the scheduler assigns to each bearer, credits, whichare equivalent to the minimum bit-rate associated with the bearer forthe period T. The assignment can be added to any positive or negativecredits carried forward from previous time intervals.

As shown in FIG. 1, credits in a scheduler are controlled using a timer.A clock 1 is started, which continues whilst an elapsed time T is notexceeded 2. When the elapsed time is exceeded 3, the clock is reset 4and a credit is incremented 5 by T×the minimum bit rate. A check is madeas to whether the accumulated credit has reached 6 a maximum credit. Ifthe credit has not exceeded 7 the maximum, the timer and incrementingsteps continue, if the credit is equal 8 to the maximum credit, thechecking stops 9.

In the method, there is signalling from a cellular network, e.g. from abase station or a network control entity, to a mobile device e.g. a 3GPPLTE UE, of a time period, here called T. The regulating time parameter Tis sent in a message 10 to the UE 11 by the eNode B 12, as illustratedin FIG. 2.

The flow diagram of FIG. 3 illustrates the method in more detail. Agrant is received 20 for a transmission period and initially theavailable grant is equal to the grant. For each bearer, in priorityorder 21, a check is made 22 as to whether the credit is greater than 0.If not 23, then checks continue, if the credit is 24 greater than 0,then a check 25 is made as to whether the data buffer is non-empty. Ifnot 26, the checks 21 continue, if the data buffer is 27 non-empty, thena check 28 is made as to whether the grant is less than or equal to thecredit. If the grant is 29 less than the credit, then a check 30 is madeas to whether the grant is less than or equal to the buffer. If not 31,then 32 add buffer to transmit block, reduce available grant by bufferand reduce credit by buffer. If the available grant is 33 less than orequal to the buffer, then 34 fill the transmit block from the buffer bythe available grant and reduce the credit by the available grant.

Where the available grant is not 35 less than or equal to the credit, acheck 36 is made of whether the credit is less than or equal to thebuffer. If not 37, then 38 add the buffer to the transmit block, reducethe available grant by the buffer and reduce the credit by the buffer.If the credit is 39 less than or equal to the buffer, then add from thebuffer to the transmit block to the value of the credit incremented andif necessary, up to the next SDU boundary, provided that this does notexceed the credit by more than a specified margin, or exceed theavailable grant; decrement the credit by the size of the included data,and decrement the available grant by the size of the included data. Thenreturn to the check stage 21. In both cases where the buffer is added tothe transmit block, the checking cycle 21 continues.

In each TTI for which the scheduler has resources available, thescheduler takes data from that bearer which has highest priority, anon-empty transmission buffer and positive credit. Accepting data from abearer reduces its credit by an amount that is equivalent to the volumeof data transmitted. This mechanism ensures that if data arrives for ahigher priority bearer that has positive credit within the TTI, then thedata on the higher priority bearer will be served in the next TTI.

Unused minimum capacity credit can be added to the credit assigned inthe next time interval T i.e. carried forward. To prevent carriedforward credit resulting in temporary starvation of lower prioritybearers, then an upper limit is set for carry forward credit e.g. notmore than ‘r’ times the capacity assigned to the bearer in interval T.Alternatively, all minimum capacity for time T is served before carryforward capacity.

If all bearers with a minimum bit rate credit assignment have eitherzero credits or empty buffers then any available resources are assignedto bearers so as to fulfill the criteria that are to apply when theminimum capacity requirements have been met.

In the method, when accepting data from a bearer in step 2 above, thescheduler accepts a larger packet than the bearer's minimum credit wouldallow, resulting in the bearer's credit becoming negative. This can bedone if, for example, it is efficient to do so from a radio point ofview, e.g. if it avoids segmenting the packet.

The negative credit that results may be carried forward into the nexttime period T. An upper limit may be set on the excess credit permitted,for example, not greater than that to be assigned in ‘n’ intervals T.

Thus the time interval T forms a basis for ensuring that the resourceallocation function in the UE meets the minimum bit-rate criteria thatit is required to implement. Similar procedures can be used to ensurethat the resource allocation function does not exceed a maximum bit-raterequirement for a particular bearer. If the mobile network defines thetime interval T that is to be used within the UE then a consistentbehavior is possible for each UE.

Signalling of the time period, T to the UE allows the UE to control theallocation of resources to particular bearers, so that it can ensurethat target minimum bit rates are achieved before resources are used forother purposes e.g. via a credit system similar to that described above;or enables the UE to control the allocation of resources to particularbearers so that it can ensure that target maximum bit rates are notexceeded e.g. via a credit system similar to that described above.Alternatively, the time period is signalled to and used by a testingentity to verify that the UE has met the requirements placed on it toensure resources are allocated in the way that the network hasspecified.

The network can set a guaranteed bit rate and notify the UE of this,then the UE indicates what buffer space it has available, withoutspecifying the priority level of this. A percentage permitted overloadfor any service that has a minimum guaranteed bit rate can be signalledas part of the bearer parameters during radio bearer configuration. Themethod is UE specific, so when a service is set up, a suitable bearer isset up, e.g. VOIP for voice, during the configuration stage. Anybackground data only gets best efforts service, but no minimum bit rateguarantee.

The system also includes permanent or removable storage, such asmagnetic and optical discs, RAM, ROM, etc. on which the process and datastructures of the present invention can be stored and distributed. Theprocesses can also be distributed via, for example, downloading over anetwork such as the Internet. The system can output the results to adisplay device, printer, readily accessible memory or another computeron a network.

A description has been provided with particular reference to preferredembodiments thereof and examples, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the claims which may include the phrase “at least one of A, B and C”as an alternative expression that means one or more of A, B and C may beused, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69USPQ2d 1865 (Fed. Cir. 2004).

1. A method of scheduling packets in a mobile communication system,comprising: assigning a positive credit to each bearer of a terminal ineach time period for which resources are available; and accepting datapackets from a bearer in accordance with predefined constraints ofpriority, buffer content and credit, where the data packets acceptedinclude a data packet having a size that exceeds an available credit byan amount in a predefined range, and wherein, when accepting data fromthe bearer, the credit of the bearer is reduced by the size of theaccepted data packet, and when the size of the accepted data packetexceeds the credit of the bearer, an amount by which the credit isexceeded is carried forward to a next time period as a negative credit,and a credit to be assigned to the bearer in the next time period isadded to the negative credit carried forward.
 2. A method according toclaim 1, wherein the predefined constraints are for a highest priority,non-empty buffer and positive credit.
 3. A method according to claim 2,further comprising adding the buffer content to a data block fortransmission to one of a limit of the credit and a next service dataunit boundary, if the bearer satisfies the predefined constraints.
 4. Amethod according to claim 3, wherein the predefined range is set toaccommodate un-segmented packets.
 5. A method according to claim 4,further comprising carrying forward, any amount by which the credit isexceeded, to the next time interval as a negative credit.
 6. A methodaccording to claim 5, further comprising carrying forward, any amount bywhich the credit is unused, to the next time interval as a positivecredit.
 7. A method according to claim 6, wherein an upper limit is setfor carried forward positive and negative credit.
 8. A method accordingto claim 7, wherein the upper limit is an integer multiple of an amountof credit allocated to a bearer in one time interval.
 9. A methodaccording claim 8, further comprising serving, in any time interval forwhich resources are available, each bearer at a minimum data ratethereof in preference to bearers using carried forward credit.
 10. Amethod according to claim 9, wherein the mobile communication system isa universal mobile telecommunication system.
 11. A method according toclaim 5, wherein an upper limit is set for carried forward negativecredit.
 12. A method according to claim 11, wherein the upper limit isan integer multiple of an amount of credit allocated to a bearer in onetime interval.
 13. A method of controlling allocation of resources in amobile device of a communication system, comprising: signalling from anetwork control entity to the mobile device a time period and using thetime period, including a positive credit for each bearer of a pluralityof terminals in each time period for which resources are available; andaccepting data packets from the mobile device in accordance withpredefined constraints of priority, buffer content and credit, where thedata packets accepted include a data packet having a size that exceedsan available credit by an amount in a predefined range, and wherein,when accepting data from the bearer, the credit of the bearer is reducedby the size of the accepted data packet, and when the size of theaccepted data packet exceeds the credit of the bearer, an amount bywhich the credit is exceeded is carried forward to a next time period asa negative credit, and a credit to be assigned to the bearer in the nexttime period is added to the negative credit carried forward.
 14. Amethod according to claim 13, wherein the mobile device is one of theterminals of the communication system, or a testing entity used toverify operation of the terminals.
 15. A method according to claim 13,further comprising one of transferring a timer value to the mobiledevice across a radio interface in radio resource control signaling sentby the network control entity, or setting the timer value internally inthe mobile device to a specified value.
 16. A method according to claim13, further comprising using a timer within the mobile device to ensurethat the mobile device meets minimum bit rate and/ or maximum bit ratecriteria set for each bearer.
 17. A method according to claim 16,further comprising using the timer in a mobile device test regime as ameasure of the interval in which the mobile device must meet the minimumand/ or maximum bit rates set for each bearer.
 18. A method according toclaim 1, wherein the data packets from the bearer are transmitted havingthe size that exceeds the available credit by the amount in thepredefined range.